I have a few guidelines I like to use, like whether I am doing the work or delegating it to others, how big the work is, what deliverable it relates to etc.
The odour strategy main deliverable will be a document which I write. To reduce emissions, there will need to be a cover installed on a reactor and a chemical dosing plant installed, and chemical dosing trials (of which a report needs to be done). The first two are projects managed by others but which I play a key technical role, the third I am project managing and doing the work. All these three I consider to be separate projects, because they are very large, and there are very significant deliverables quite different from the strategy. I tend to always make delegated work a separate project, because it makes the filing easier for myself and the other person. These are fairly straightforward to make separate projects.
Then there are some pieces of work that are a bit harder to decide. For example, after getting confused with some of the dosing trials, I decided that I really needed to do a literature review to understand more about odour and how the chemicals help. And then I think I really need to know a lot more about odour in general and should broaden this to other odour information. This will be done by me, and could be part of the strategy or part of the chemical dosing. I could make it a stand alone project, but I only would do so if I was going to have a separate deliverable, eg if I decided that I was going to do a literature review report, then I would make it a separate project. But in the end I decided not to, and put the literature review on chemical dosing as a subproject of the chemical dosing trial work, and general literature review as a subproject of the strategy.
In practice I keep all my project plans (just for myself) in the one word document for the same area of focus. ie, the above projects will all be listed in the "Odour Plan" document. So its easy to find the info by searching, and easy to make new projects.
If I'm collaborating with others and they also want to see the plan, I'll keep a separate tidier version for everyone that only gets updated prior to meetings or agreed timelines, the one I keep for myself is 'live' and gets updated very often, usually I'll add to it or update it a little whenever I work on the projects. That way I can write any way I like in my version, and only have to worry about doing 'professional' writing in the group version.